ethtool: stats: clarify the initialization to ETHTOOL_STAT_NOT_SET

Ido suggests we add a comment about the init of stats to -1.
This is unlikely to be clear to first time readers.

Suggested-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>